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SNORT_INLINE - The Easy Tutorial - Bridge

Snort_Inline Bridge
Last Change : Apr 26 2007 french flagenglish flag


Tool
Install
Ergonomy
Forum



Details What is Snort_Inline?
Screenshots
Prerequisites
Installation
Oinkmaster - Snort Rules
Oinkmaster - Bleeding Rules
Run Snort_Inline
BASE
Bridging




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After all our efforts to configure Snort_Inline, we must be careful to put our brand new IPS at the right place in the network.

A Snort IDS just needs to be connected to a switch port where it receives the traffic coming from a mirrored port. See how to configure port mirroring.

For our IPS, the network configuration above cannot be used because Snort_Inline must check the packets before deciding to drop or accept them. It must be placed along the network link we want to analyze and configured as a bridge.
So you must have a minimum of two networks interfaces, three of them is better because you can dedicate a port to the IPS management.

bridge debian ubuntu IPS snort_inline

Load the bridge kernel module.

#modprobe bridge
Install the tool to manage bridges:

#apt-get install bridge-utils
Configure the networking (/etc/network/interfaces file).
If you have a network interface dedicated to the bridge management, choose case 1, otherwise (ie: you have only 2 network interfaces) choose case 2.

CASE 1 CASE 2
 # /etc/network/interfaces file
 #
 # Loopback interface
 auto lo
      iface lo inet loopback

 #
 # Configure the bridge
 auto br0
 iface br0 inet manual

    # Ports you want to add to your bridge
    bridge_ports eth0 eth1
    # Time to wait before loading the bridge
    bridge_maxwait 0
 #
 # Configure the bridge management
 # interface.
 auto eth2
 iface eth2 inet static
      address 192.168.1.22
      netmask 255.255.255.0
      broadcast 192.168.1.255
      gateway 192.168.1.1
 # /etc/network/interfaces file
 #
 # Loopback interface
 auto lo
      iface lo inet loopback

 #
 # Configure the bridge
 auto br0
 iface br0 inet static
      address 192.168.1.22
      netmask 255.255.255.0
      broadcast 192.168.1.255
      gateway 192.168.1.1

    # Ports you want to add to your bridge
    bridge_ports eth0 eth1
    # Time to wait before loading the bridge
    bridge_maxwait 0
 
 
 
 
 
Restart the networking service.

#/etc/init.d/networking restart
* Reconfiguring network interfaces...



You can watch the logs with the following command:

#tail -f /var/log/syslog
Below is given the log output during the networking service restart.

ubuntu kernel: br0: port 2(eth1) entering disabled state
ubuntu kernel: br0: port 1(eth0) entering disabled state
ubuntu dhclient: receive_packet failed on eth0: Network is down
ubuntu kernel: device eth0 left promiscuous mode
ubuntu kernel: audit(1175176752.661:116): dev=eth0 prom=0 old_prom=256 auid=4294967295
ubuntu kernel: br0: port 1(eth0) entering disabled state
ubuntu kernel: eth1: Setting promiscuous mode.
ubuntu dhclient: receive_packet failed on eth1: Network is down
ubuntu dhclient: receive_packet failed on eth1: Network is down
ubuntu kernel: device eth1 left promiscuous mode
ubuntu kernel: audit(1175176752.669:117): dev=eth1 prom=0 old_prom=256 auid=4294967295
ubuntu kernel: br0: port 2(eth1) entering disabled state
ubuntu kernel: device eth0 entered promiscuous mode
ubuntu kernel: audit(1175176752.741:118): dev=eth0 prom=256 old_prom=0 auid=4294967295
ubuntu kernel: ADDRCONF(NETDEV_UP): eth0: link is not ready
ubuntu kernel: e100: eth0: e100_watchdog: link up, 100Mbps, half-duplex
ubuntu kernel: device eth1 entered promiscuous mode
ubuntu kernel: audit(1175176752.769:119): dev=eth1 prom=256 old_prom=0 auid=4294967295
ubuntu kernel: ACPI: PCI Interrupt 0000:01:0e.0[A] -> GSI 22 (level, low) -> IRQ 185
ubuntu kernel: eth1: setting full-duplex.
ubuntu kernel: eth1: Setting promiscuous mode.
ubuntu last message repeated 6 times
ubuntu kernel: br0: port 2(eth1) entering learning state
ubuntu kernel: br0: port 1(eth0) entering learning state
ubuntu kernel: ADDRCONF(NETDEV_CHANGE): eth0: link becomes ready
ubuntu kernel: br0: no IPv6 routers present
ubuntu kernel: eth1: no IPv6 routers present
ubuntu kernel: eth0: no IPv6 routers present
ubuntu kernel: br0: topology change detected, propagating
ubuntu kernel: br0: port 2(eth1) entering forwarding state
ubuntu kernel: br0: topology change detected, propagating


The network interfaces statistics (case 2, see above)
The bridge MAC address is the smallest MAC address of its members.

#ifconfig
br0     Link encap:Ethernet HWaddr 00:00:11:22:33:44
         inet addr:192.168.1.22 Bcast:192.168.1.255 Mask:255.255.255.0
         inet6 addr: fe80::202:55ff:fe13:9d05/64 Scope:Link
         UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
         RX packets:1061 errors:0 dropped:0 overruns:0 frame:0
         TX packets:878 errors:0 dropped:0 overruns:0 carrier:0
         collisions:0 txqueuelen:0
         RX bytes:723663 (706.7 KiB) TX bytes:115514 (112.8 KiB)

eth0    Link encap:Ethernet HWaddr 00:00:11:22:33:44
         inet6 addr: fe80::202:55ff:fe13:9d05/64 Scope:Link
         UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
         RX packets:16738 errors:0 dropped:0 overruns:0 frame:0
         TX packets:6824 errors:0 dropped:0 overruns:0 carrier:0
         collisions:10 txqueuelen:1000
         RX bytes:5782167 (5.5 MiB) TX bytes:704700 (688.1 KiB)

eth1    Link encap:Ethernet HWaddr 00:99:aa:bb:cc:dd
         UP BROADCAST MULTICAST MTU:1500 Metric:1
         RX packets:36605 errors:0 dropped:0 overruns:0 frame:0
         TX packets:1842 errors:0 dropped:0 overruns:0 carrier:14
         collisions:0 txqueuelen:1000
         RX bytes:9446601 (9.0 MiB) TX bytes:160001 (156.2 KiB)
         Interrupt:185 Base address:0x4f80

lo      Link encap:Local Loopback
        inet addr:127.0.0.1 Mask:255.0.0.0
        inet6 addr: ::1/128 Scope:Host
        UP LOOPBACK RUNNING MTU:16436 Metric:1
        RX packets:8393 errors:0 dropped:0 overruns:0 frame:0
        TX packets:8393 errors:0 dropped:0 overruns:0 carrier:0
        collisions:0 txqueuelen:0
        RX bytes:1077749 (1.0 MiB) TX bytes:1077749 (1.0 MiB)


Use the brctl command to see the bridge members.

#brctl show
bridge name   bridge id                     STP enabled       interfaces
br0               8000.000255139d05       no                   eth1
                                                                         eth0


Add a cron job to load the bridge kernel module each time your Linux boots.

#crontab -e -u root
@reboot root lsmod | grep bridge >/dev/null || /sbin/modprobe bridge;



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